As Jim says, the valve timing is designed so the flow to the side of the valve that will be the "out" or pressure side is there before the "in" or return side opens. This prevents the cylinder from moving slightly in the wrong direction as the valve is moved. Because the "in" side is not open yet, or less open than the "out" side, the pressure is relieved over the relief valve, and they have tendancy to vibrate when barely opening or "cracking". Some relief valves can be pretty noisy or vibrate badly when cracking. You are lucky the hydraulic systems of that era are fairly low pressure.... a 5000 psi relief valve can have a really loud squeal :shock:
Part of the noise is induced by the gear teeth creating pulses, and in some systems they set up a resonant frequency that aggravates the noise. You may find the noise is worse at some engine RPMs than others.
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Today's Featured Article - History of the Nuffield Tractor - by Anthony West. The Nuffield tractor story started in early 1945. The British government still reeling from the effects of the war on the economy, approached the Nuffield organization to see if they would design and build an "ALL NEW" British built wheeled tractor, suitable for both British and world farming.
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